Method for manufacturing reduced iron

ABSTRACT

Provided is a process for manufacturing reduced iron by heating agglomerates, said process being capable of enhancing the yield of reduced iron and thus improving the productivity thereof. A process for manufacturing reduced iron which includes a step for agglomerating a mixture that comprises an iron oxide source, a carbonaceous reducing agent and a melting point regulator and a step for heating the obtained agglomerates to reduce the iron oxide contained in the agglomerates, wherein the agglomerates contain at least 1 mass % of a silicate mineral having a solidus temperature of 1300° C. or lower.

TECHNICAL FIELD

The present invention relates to a method for heating an agglomerate including an iron oxide source such as iron ore or iron-making dust, and a carbonaceous reducing agent such as a carbon material, and manufacturing a reduced iron.

BACKGROUND ART

As an iron-making process using iron ore as a raw material, the blast furnace—converter method is the main stream. However, with this method, pre-treatments of raw materials such as carbonization of coal and sintering of iron ore are required to be performed. Further, in order to enjoy the scale merit, in recent years, there has been a growing trend toward a larger-size blast furnace or converter, resulting in reduction of flexibility or the production resilience to resources.

Whereas, the iron-making process is required to control the emission amount of CO₂ gas from the viewpoint of environmental conservation. However, the blast furnace—converter method described above is a so-called indirect iron-making method in which in a blast furnace, iron ore is reduced to manufacture a high-carbon molten iron, and the resulting molten iron is decarbonized in a converter, thereby to manufacture a steel. For this reason, as compared with a direct iron-making method in which iron ore is reduced to directly manufacture a steel, the amount of CO₂ gas generated is larger. Under such circumstances, in recent years, from the viewpoint of controlling the emission amount of a CO₂ gas, the direct iron-making method has been reconsidered.

As the direct iron-making method, a MIDREX method has been known in the related art. However, with the MIDREX method, a large amount of natural gas is used for reduction of iron ore. For this reason, the locational conditions of plants are unfavorably limited to the production regions of a natural gas.

Under such circumstances, in recent years, attention has been paid to a reduced iron manufacturing process using coal relatively easily available as a carbonaceous reducing agent in place of a natural gas. The reduced iron manufacturing process includes: charging an agglomerate including an iron oxide source, a carbonaceous reducing agent, and a melting point adjusting agent into a shifting hearth type heating furnace (e.g., rotary furnace hearth), and heating the agglomerate with gas transfer heat or radiant heat by a heating burner in the furnace, thereby to reduce the iron oxide, resulting in an agglomerate metal iron. The reduced iron manufacturing process has advantages such as being capable of directly using a powdery iron ore, being capable of performing high-speed reduction of iron oxide in the iron ore because the iron ore and the reducing agent are placed in proximity to each other during reduction, and being capable of readily adjusting the carbon content in the product obtained by reduction, other than being based on coal.

However, in the reduced iron obtained in the reduced iron manufacturing process, iron oxide sources such as iron ore used as the raw material, and gangue components such as CaO, SiO₂, and Al₂O₃ included in a carbonaceous reducing agent such as coal are mixed as slugs. Accordingly, the quality of the reduced iron is unfavorably degraded.

For this reason, the obtained reduced iron is required to be molten in, for example, an electric furnace, thereby to separate and remove the slugs. However, an increase in the amount of slugs contained in the reduced iron results in a reduction of the yield for refining. For this reason, the reduced iron is required to have a low slug content, and to have a high iron quality.

In order to enhance the iron quality of the reduced iron, it is effective to use an iron oxide source with a high iron quality as a raw material. However, while the production amount of steel has been increasing on a worldwide basis, the amount of high-quality iron ore mined tends to decrease. For this reason, the price of the high-quality iron ore is predicted to increase. Under such circumstances, it is necessary to use an iron oxide source with a low iron quality as a raw material.

However, when a low-quality iron ore is used as a raw material, molten slugs increase. This hinders the heat transfer to the agglomerate, resulting in the reduction of the productivity of metal iron. Under such circumstances, Patent Literature 1 proposes the following method: from not only an iron oxide with a high iron component content but also an iron ore or the like with a relatively lower iron component content, a metal iron with a very high iron purity is manufactured as a solid metal iron or molten metal iron. With this method, by heating reduction, reduction is allowed to proceed until the iron oxide substantially ceases to be present in the inside. In addition, the resulting slugs including gangue components and metal iron are agglomerated in the inside, and the metal iron and the slugs are molten and separated.

As a method in which melting and separating of metal iron and slugs are performed for a short time, thereby to manufacture metal iron with a high quality, Patent Literature 2 is proposed. This literature discloses the following technology: the content ratios of CaO, SiO₂, and Al₂O₃ included in the raw material for a carbon composite iron oxide formed body are adjusted, thereby to reduce the melting point of the resulting slug including the gangue components to 1400° C. or less; as a result, melting and separating of the metal iron and the slugs are promoted. Further, in this literature, it is described that CaSiO₃ is added in order to adjust the content ratios of CaO, SiO₂, and Al₂O₃ in the raw material, and thereby to reduce the melting point of the resulting slugs.

CITATION LIST Patent Literatures

[Patent Literature 1] Japanese Unexamined Patent Publication No. 9-256017

[Patent Literature 1] Japanese Unexamined Patent Publication No. 11-199911

SUMMARY OF INVENTION Technical Problem

In accordance with the Patent Literature 1, a metal iron with a high iron purity can be obtained as a solid metal iron or a molten metal iron. However, there has been room for further improvement in recovery (yield) of a metal iron.

On the other hand, in the Patent Literature 2, it is described that CaSiO₃ is added to an agglomerate. However, the solidus temperature of CaSiO₃ is 1545° C. For this reason, the effect of lowering the melting point of the slug is weak, so that a metal iron and slugs could not have been molten and separated sufficiently.

The present invention was completed in view of the foregoing circumstances. It is an object thereof to provide a method for enhancing the yield and improving the productivity in heating an agglomerate and manufacturing a reduced iron.

Solution to Problem

A method for manufacturing a reduce iron in accordance with the present invention which could solve the problem has the gist in the following: the method includes: a step of agglomerating a mixture including an iron oxide source, a carbonaceous reducing agent, and a melting point adjusting agent, and a step of heating a resulting agglomerate to reduce an iron oxide in the agglomerate; and as the agglomerate, the one containing a silicate mineral with a solidus temperature of 1300° C. or less in an amount of 1 mass % or more is used.

It is preferable that the silicate mineral contains a volatile matter. Representative examples of such a mineral having a solidus temperature of 1300° C. or less and containing a volatile matter may include those of an amphibole group such as actinolite, cummingtonite, and grunerite.

In the present invention, as the iron oxide source, the one containing a silicate mineral with a solidus temperature of 1300° C. or less may be used. As the melting point adjusting agent, a silicate mineral with a solidus temperature of 1300° C. or less may be used. Both may be used.

Advantageous Effects of Invention

In accordance with the present invention, for heating an agglomerate, and manufacturing a reduced iron, there is used an agglomerate containing a silicate mineral with a solidus temperature of 1300° C. or less in an amount of 1 mass % or more. For this reason, molten slugs are formed rapidly. As a result, the agglomeration of the reduced iron is promoted, so that the yield is improved, resulting in a higher productivity.

DESCRIPTION OF EMBODIMENTS

In heating an agglomerate, and manufacturing a reduced iron, the present inventors have conducted a close study for improving the yield of the reduced iron, and enhancing the productivity of the reduced iron. As a result, it has been found as follows: when, as the agglomerate, the one containing a silicate mineral with a solidus temperature of 1300° C. or less in an amount of 1 mass % or more is used, the silicate mineral is molten at an early stage; with this site as a starting point, melting of gangues and fluxes advances at once; accordingly, molten slugs are formed rapidly. It has been found that the molten slugs promote agglomeration among reduced irons, which can improve the yield of reduced irons, and further the productivity thereof. Thus, the present invention was completed.

Below, after describing how the present invention is completed, a description will be given to the distinctive portions of the present invention.

As described above, in the Patent Literature 2, it is described that CaSiO₃ is added to an agglomerate. However, the actual manufacturing process must be performed in a limited time. For this reason, the amount of molten slugs is largely affected by not only the melting point in equilibrium determined from the composition, but also the forms of the melting point adjusting agent added to the agglomerate, and the mineral phase in gangues contained in the raw material.

Herein, melting of gangues contained in the agglomerate advances at once upon formation of the melt solution serving as the starting point. Accordingly, in order to form molten slugs at an early stage, it is important to form the melt solution serving as a starting point rapidly. In the reduced iron manufacturing process, commonly used slugs are CaO—SiO₂—FeO_(x) series. Of the mineral phases included in the agglomerate, quartz represented by SiO₂ has a high melting point, and further, has a property of being resistant to undergo solid phase diffusion with CaO or FeO, resulting in slow melting. Thus, in the timing at which molten slugs are formed, melting of SiO₂ often becomes rate limiting. For this reason, preferably, SiO₂ is not quartz, but has a mineral phase in another form. Especially, it is considered desirable for SiO₂ to combine with CaO, FeO, and the like.

Under such circumstances, the present inventors conducted a study in order to promote melting of SiO₂ contained in the agglomerate. This has revealed that use of an agglomerate containing a silicate mineral with a solidus temperature of 1300° C. or less in an amount of 1 mass % or more can promote melting of SiO₂.

Below, the present invention will be described.

A method for manufacturing a reduced iron in accordance with the present invention is characterized by including:

-   -   a step of agglomerating a mixture including an iron oxide         source, a carbonaceous reducing agent, and a melting point         adjusting agent (which may be hereinafter referred to as an         agglomerating step), and     -   a step of heating a resulting agglomerate to reduce an iron         oxide in the agglomerate (which may be hereinafter referred to         as a heating reduction step),     -   and is characterized in that as the agglomerate, the one         containing a silicate mineral with a solidus temperature of         1300° C. or less in an amount of 1 mass % or more is used.

Agglomerating Step

In the agglomerating step, a mixture including an iron oxide source, a carbonaceous reducing agent, and a melting point adjusting agent is agglomerated, thereby to manufacture an agglomerate containing a silicate mineral with a solidus temperature of 1300° C. or less in an amount of 1 mass % or more.

First, a description will be given to the silicate minerals characterizing the present invention. Silicate minerals are one kind of rock-forming minerals, and minerals including SiO₂.

There are various silicate minerals. However, the present invention is characterized in that an agglomerate containing a silicate mineral with a solidus temperature of 1300° C. or less of silicate minerals is used. SiO₂ included in a silicate mineral with a solidus temperature of 1300° C. or less is present in the mineral phase with a low melting point. For this reason, SiO₂ can be molten rapidly. As a result, a melt solution is formed rapidly, and hence molten slugs are formed rapidly. This promotes the agglomeration of reduced iron, resulting in a higher yield of the reduced iron.

The reasons why the upper limit of the solidus temperature of the silicate mineral is defined at 1300° C. are as follows: the maximum temperature for heating the agglomerate to reduce the iron oxide in the agglomerate is about 1300 to 1500° C.; and during the reduction of the iron oxide, the silicate mineral is molten to form a melt solution.

The solidus temperatures in the case where the silicate minerals have ternary or less system mineral phases are described in literatures such as Verlag Stahlen GmbH, SLAG ATLAS 2nd Ed. (Germany, 1995), and Phase Diagram for Ceramists. On the other hand, the solidus temperatures in the case where the silicate minerals have quaternary or more system mineral phases can be calculated using thermodynamics software such as Fact Sage (Ver 6.3) using FT-OXIDE DB.

Incidentally, the agglomerate containing a silicate mineral with a solidus temperature of 1300° C. or less means an agglomerate containing a silicate mineral with a solidus temperature as low as 1300° C. or less of various silicate minerals. As described later, a silicate mineral may be contained in the iron oxide source included in the agglomerate, or a silicate mineral may be used as a melting point adjusting agent forming the agglomerate. When the composition of the agglomerate is measured, there may be properly present a silicate mineral with a solidus temperature of 1300° C. or less.

It is important that the ratio of the silicate mineral with a solidus temperature of 1300° C. or less included in the agglomerate is 1 mass % or more. When the ratio of the silicate mineral with a solidus temperature of 1300° C. or less is less than 1 mass %, a melt solution cannot be formed rapidly to enhance the yield of the reduced iron. Therefore, the ratio of a silicate mineral with a solidus temperature of 1300° C. or less is preferably 1.5 mass % or more, and more preferably 2 mass % or more. The upper limit of the ratio of a silicate mineral with a solidus temperature of 1300° C. or less has no particular restriction. However, when the ratio of a silicate mineral contained in the agglomerate is too high, the mixing amount of an iron ore included in the agglomerate decreases. Accordingly, the iron content in the agglomerate decreases. Thus, even when the yield is improved, the productivity is reduced. For this reason, the ratio is preferably, for example, 20 mass % or less.

Incidentally, in the present invention, as described above, it is essential only that the ratio of a silicate mineral with a solidus temperature of 1300° C. or less of silicate minerals is 1 mass % or more. For example, in addition to the silicate mineral with a solidus temperature of 1300° C. or less, further, a silicate mineral with a solidus temperature of more than 1300° C. may be contained in the agglomerate in such a range as not to impair the advantageous effects of the present invention.

The ratio of a silicate mineral with a solidus temperature of 1300° C. or less contained in the agglomerate can be measured in the following manner. The components are measured by X-ray diffraction (XRD), and analyzed by a semiquantitative method.

In the present invention, the silicate mineral preferably contains a volatile matter. When a silicate mineral containing a volatile matter is heated, the volatile matter is liberated during heating, so that the silicate mineral becomes porous. As a result, the surface area increases, resulting in a further improvement of the melting speed. Accordingly, agglomeration of reduced iron particles is promoted, so that the yield becomes higher, resulting in an improvement of the productivity.

As the volatile matters, there is preferably contained one or more selected from the group consisting of a hydroxy group, a carbonate group, and crystalline water.

When the solidus temperature of a silicate mineral containing a volatile matter is measured, it is essential only to measure the solidus temperature of the residue after volatilization and removal of the volatile matter. For example, in the case of kaolinite [Al₄Si₄O₁₀(OH)₈], a hydroxy group is contained as a volatile matter. For this reason, it is essential only to measure the solidus temperature in Al₄Si₄O₁₀ after removal of the hydroxy group.

The silicate mineral with a solidus temperature of 1300° C. or less is preferably of the amphibole group. As the amphibole group, there is preferably used at least one or more selected from the group consisting of actinolite [Ca₂(Fe, Mg)₅Si₈O₂₂(OH)₂], cummingtonite [(Fe, Mg)₇Si₈O₂₂(OH)₂], and Grunerite [Fe₇Si₈O₂₂(OH)₂].

Of the amphibole group, actinolite, cummingtonite, and grunerite are minerals in which SiO₂ combines with FeO and CaO, and has a solidus temperature as low as 1300° C. or less, and further has a hydroxy group. For this reason, the hydroxy group is liberated during heating, resulting in a porous structure, which is more likely to be molten. Accordingly, at an early stage, molten slugs are formed, resulting in an improvement of agglomerating property of metal iron. This can in turn enhance the yield.

Further, the amphibole group is a mineral having a small content of T. Fe, and included in a large amount in BIF (Banded Iron Formation) which has not often used as an iron oxide source, and hence is readily available.

The determination of the amphibole group is difficult only by X-ray diffraction (XRD). However, use of X-ray diffraction (XRD) and a scanning electron microscope (SEM) in combination enables the determination. Namely, identification may be performed in the following manner: the amphibole is determined by the mapping function in SEM observation; then, composition analysis is carried out by EDS and the like included in SEM; in this step, when CaO is detected, the sample is identified as actinolite, whereas, when CaO is not detected, but MgO is detected, the sample is identified as cummingtonite.

Then, others than silicate minerals will be described.

As the iron oxide sources, specifically, there may be used iron oxide-containing substances such as iron ore, iron sand, iron-making dust, non-iron refining residue, and iron-making waste. Further, in the present invention, there may be used an iron oxide source containing a silicate mineral with a solidus temperature of 1300° C. or less.

As the carbonaceous reducing agent, there can be used, for example, coal or coke.

It is essential only that the carbonaceous reducing agent contains carbon in an amount capable of reducing the iron oxide included in the iron oxide source. Specifically, it is essential only that carbon is contained in an amount in the range of 0 to 5 mass % larger than or 0 to 5 mass % smaller than (i.e., ±5 mass % of) the carbon content capable of reducing the iron oxide contained in the iron oxide source.

Into the mixture including the iron oxide source and the carbonaceous reducing agent, a melting point adjusting agent is required to be further mixed.

The melting point adjusting agent means a substance having an action of lowering the melting point of the gangue in an iron oxide source, or the ash content in a carbonaceous reducing agent. Namely, mixing of a melting point adjusting agent in the mixture can affect the melting points of other components (particularly, gangue) than iron oxide contained in the agglomerate, thereby, for example, to lower the melting points. As a result, melting of the gangue is promoted, so that molten slugs are formed. At this step, a part of the iron oxide is molten in the molten slugs, and is reduced in the molten slugs, resulting in a metal iron. The metal iron formed in the molten slugs comes in contact with a metal iron reduced still in a solid state, thereby to be agglomerated as a solid reduced iron.

As the melting point adjusting agents, there can be used, for example, a CaO supplying substance, a MgO supplying substance, an Al₂O₃ supplying substance, and a SiO₂ supplying substance. Further, in the present invention, as at least a part of the melting point adjusting agent, there may be used a silicate mineral with a solidus temperature of 1300° C. or less.

As the CaO supplying substances, there can be used at least one selected from the group consisting of, for example, CaO (quick lime), Ca(OH)₂ (slaked lime), CaCO₃ (limestone), and CaMg(CO₃)₂ (dolomite). As the MgO supplying substances, there may be mixed at least one selected from the group consisting of, for example, a MgO powder, Mg-containing substances extracted from natural ores, seawater, and the like, and MgCO₃. As the Al₂O₃ supplying substances, there can be mixed, for example, an Al₂O₃ powder, bauxite, boehmite, gibbsite, and diaspore. As the SiO₂ supplying substances, there can be used, for example, a SiO₂ powder and silica sand.

Into the agglomerate, a binder and the like may be further mixed as other components than an iron oxide source, a carbonaceous reducing agent, and a melting point adjusting agent.

As the binders, there can be used, for example, polysaccharides (e.g., starches such, as corn starch and wheat flour).

The iron oxide source, the carbonaceous reducing agent, and the melting point adjusting agent are preferably pre-ground before mixing. For example, grinding is recommendably performed such that the average particle size of the iron oxide source is 10 to 60 μm, such that the average particle size of the carbonaceous reducing agent is 10 to 60 μm, and such that the average particle size of the the melting point adjusting agent is 5 to 90 μm.

The means for grinding the iron oxide source and the like has no particular restriction, and known means can be adopted. There may be used, for example, a vibrating mill, a roll crusher, and a ball mill.

Mixing of the raw materials is carried out by a mixer in a rotary container shape or a fixed container shape. The types of the mixer include: a rotary cylindrical shape, a double cone shape, a V shape and the like as the rotary container shape; and a case in which a rotary vane (such as a spade) is provided in a mixing tank as the fixed container shape. However, the system has no particular restriction.

As agglomerators for agglomerating the mixture, there can be used, for example, a pan-shaped granulator (disk-shaped granulator), a cylindrical granulator (drum-shaped granulator), and a twin roll type briquette forming machine.

The shape of the agglomerate has no particular restriction. It does not matter if forming is carried out by any of pellet, briquette, and extrusion.

Heating Reduction Step

In a heating reduction step, the agglomerate obtained in the agglomerating step is heated, and the iron oxide in the agglomerate is reduced, thereby to manufacture a reduced iron.

Heating of the agglomerate may be performed by, for example, an electric furnace, or a shifting hearth type heating furnace. The shifting hearth type heating furnace is a heating furnace in which the hearth moves like a belt conveyor in the furnace. Examples thereof may include a rotary hearth furnace and a tunnel furnace.

The rotary hearth furnace is designed in a circular shape (doughnut shape) in outside shape of the hearth so that the start point and the end point of the hearth are at the same position. The iron oxide contained in the agglomerate charged onto the hearth is heated and reduced during one round in the furnace, to form a reduced iron. Therefore, in the rotary hearth furnace, a charging means for charging the agglomerate into the furnace is provided on the uppermost stream side of the rotation direction, and a discharging means is provided on the lowermost stream side of the rotation direction (in actuality, on the immediately upstream side of the charging means because of the rotation structure).

The tunnel furnace is a heating furnace in which the hearth moves in a linear direction in the furnace.

The agglomerate is preferably heated and reduced at 1300 to 1500° C. When the heating temperature is less than 1300° C., the metal iron and the slugs are less likely to be molten. As a result, a high productivity cannot be obtained. On the other hand, when the heating temperature exceeds 1500° C., the exhaust gas temperature becomes higher, resulting in larger-size exhaust gas treatment equipment. This incurs an increase in facility cost.

Prior to charging of the agglomerate into the electric furnace or the shifting hearth type heating furnace, a floor covering material such as carbonaeous and refractory ceramics is desirably laid in order to protect the hearth.

As the floor covering materials, other than those exemplified as the carbonaceous reducing agents, refractory particles may be used.

The particle size of the floor covering material is preferably 3 mm or less so as to prevent the agglomerate or the melt thereof from getting thereinto. The lower limit of the particle size is preferably 0.5 mm or more so as to prevent blowing by the combustion gas of a burner.

Others

The reduced iron obtained in the heating reduction step is discharged with by-produced slugs, the floor covering material laid, if necessary, and the like from the inside of the furnace. Screening may be performed using a sieve, a magnetic separator, or the like, thereby to collect the reduced iron.

The present application claims the benefit of priority based on Japanese Patent Application No. 2012-237276 filed on Oct. 26, 2012. The entire contents of the specification of Japanese Patent Application No. 2012-237276 are incorporated by reference in the present application.

Below, the present invention will be described more specifically by way of examples. It is naturally understood that the present invention is not limited to the following examples, and, of course, may be appropriately changed to be practiced within the scope applicable to the gist described previously and later. All of these are included in the technical range of the present invention.

EXAMPLES

A mixture including an iron oxide source, a carbonaceous reducing agent, and a melting point adjusting agent was agglomerated, thereby to manufacture an agglomerate. As the iron oxide sources, there were used iron ores A to C with the compositions shown in Table 1 below. As the carbonaceous reducing agent, there was used a carbon material with the composition shown in Table 2 below. As the melting point adjusting agents, there were used limestone, dolomite, fluorite, silica sand, and amphibole group minerals.

For the iron ores A to C and the amphibole group minerals, semiquantitative analysis using XRD was performed, thereby to measure the ratio of the mineral phase. The results are shown in Table 3 below. In Table 3 below, Fe₂O₃ represents hematite; Fe₃O₄, magnetite; αFeOOH, goethite; SiO₂, quartz; Al₂Si₂O₅(OH)₄, kaolinite; Ca₂(Mg, Fe)₅Si₈O₂₂(OH)₂, actinolite; (Fe, Mg)₇Si₈O₂₂(OH)₂, cummingtonite; CaCO₃, calcite; and Al(OH)₃, gibbsite. Out of these, Fe₂O₃, Fe₃O₄, and αFeOOH are classified as iron ores, and SiO₂, Al₂Si₂O₅(OH)₄, Ca₂(Mg, Fe)₅Si₈O₂₂(OH)₂, (Fe, Mg)₇Si₈O₂₂(OH)₂, and CaCO₃, Al(OH)₃ are classified as gangues. Of the gangues, SiO₂, Al₂Si₂O₅(OH)₄, Ca₂(Mg, Fe)₅Si₈O₂₂(OH)₂, and (Fe, Mg)₇Si₈O₂₂(OH)₂ are silicate minerals. For the silicate minerals, the solidus temperatures of mineral phases are shown together in Table 3 below. Incidentally, for the solidus temperatures of mineral phases, those with a mineral phase of ternary system or less were examined using Verlag Stahlen GmbH, and SLAG ATLAS 2nd Ed.(Germany, 1995). For those with quaternary system or more, calculation was performed by Fact Sage (Ver 6.3) using FT-OXIDE DB.

As apparent from Table 3 below, the iron ore A contains a silicate mineral with a solidus temperature of 1300° C. or less in an amount of 4 mass %. However, the iron ores B and C do not contain the silicate mineral. Whereas, the amphibole group contains a silicate mineral with a solidus temperature of 1300° C. or less in an amount of 49 mass %.

An iron oxide source, a carbonaceous reducing agent, and a melting point adjusting agent were mixed in the ratios shown in Table 4 below. Further, as a binder, wheat flower in an amount of about 1 mass %, and a proper amount of water were added. The resulting mixture was granulated using a tire type granulator into crude pellets with an average diameter of 19 mm.

The resulting crude pellets were charged into a dryer, and were heated at 180° C. for 1 hour to remove the adhesive water, and were dried. The ratio of the silicate minerals contained in the dried pellets (i.e., the amount of a silicate mineral with a solidus temperature of 1300° C. or less of the silicate minerals contained as gangues in the iron ore, and the amount of the amphibole group mineral mixed as a melting point adjusting agent) was calculated, and is shown together in Table 4 below.

Then, the dried pellets were fed to a heating furnace, and were heated at 1450° C. Thus, the iron oxide in the pellets was reduced and molten, thereby to manufacture a reduced iron. Incidentally, in order to protect the hearth of the heating furnace, a carbon material (anthracite) with a maximum particle size of 2 mm or less was laid over the hearth prior to charging of the pellets.

Further, during heating, a nitrogen gas was passed at a flow rate of 30 NL/min in the furnace, so that a nitrogen atmosphere was set in the furnace.

After completion of reduction, the sample containing a reduced iron was discharged from the inside of the furnace. The discharged substances were sieved. For sieving, a sieve with an opening of 3.35 mm was used, and oversize particles were collected.

The ratio of the mass of the oversize particles relative to the total mass of iron charged into the heating furnace is defined as a yield. The calculation results are shown in Table 4 below.

Yield (%)=(mass of oversize particles/total mass of iron charged into the heating furnace)×100

Incidentally, in the oversize particles, C, Si, Mn, and the like are also present other than Fe. For this reason, the yield may exceed 100%.

Nos. 1 and 2 are examples satisfying the requirements defined in the present invention.

No.1 is an example using the iron ore A containing a silicate mineral with a solidus temperature of 1300° C. or less (specifically, actinolite and cummingtonite) in an amount of 4 mass % as the iron oxide source. The ratio of the silicate material with a solidus temperature of 1300° C. or less occupied in the total mass of the agglomerate was 2.9 mass %. As a result, the yield of the reduced iron was 90% or more, resulting in an improvement of the productivity of the reduced iron.

No.2 is an example in which an amphibole group mineral is mixed as a melting point adjusting agent. The ratio of a silicate mineral with a solidus temperature of 1300° C. or less occupied in the total mass of the agglomerate was 1.0 mass %. As a result, the yield of the reduced iron was 90% or more, resulting in an improvement of the productivity of the reduced iron.

Nos.3 to 5 are examples not satisfying the requirements defined in the present invention, and are examples each using an agglomerate not containing a silicate mineral with a solidus temperature of 1300° C. or less. As a result, the yield of the reduced iron was less than 90%, and the productivity of the reduced iron could not be improved.

Incidentally, No.4 is an example in which silica sand is mixed as a melting point adjusting agent. The silica sand contains silicon dioxide, and is classified into a silicate mineral. However, silicon dioxide has a solidus temperature of more than 1300° C. For this reason, even when silica sand is mixed, melting of the gangue components is not promoted. Accordingly, the yield of the reduced iron could not be enhanced.

TABLE 1 Composition (mass %) Iron ore T.Fe FeO SiO₂ CaO Al₂O₃ MgO A 67.73 29.40 4.54 0.42 0.21 0.47 B 66.69 0.44 2.26 0.03 0.99 0.03 C 64.89 0.60 5.67 0.06 0.28 0.04

TABLE 2 Composition of carbon material Analytical value (mass %) Composition of ash content (mass %) Volatile matter Ash content S Others Fe₂O₃ SiO₂ CaO Al₂O₃ MgO 15.35 6.36 0.76 78.29 12.70 45.20 5.70 25.52 1.78

TABLE 3 Mineral phase Gangue Silicate minerals Tectosilicate Phyllosilicate Iron ore Silica mineral Clay mineral Fe₂O₃ Fe₃O₄ α-FeOOH SiO₂ Al₂Si₂O₅(OH)₄ (Hematite) (Magnetite) (Goethite) (Quartz) (Kaolinite) Solidus — — — 1725 1595 temperature (° C.) Iron ore A 1 90 0 4 0 (mass %) Iron ore B 94 0 1 2 0 (mass %) Iron ore C 88 2 3 6 1 (mass %) Amphibole 0 7 0 52 2 group mineral (mass %) Mineral phase Gangue Silicate minerals Inosilicate Amphibole group Others Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂ (Fe,Mg)₇Si₈O₂₂(OH)₂ CaCO₃ Al(OH)₃ (Actinolite) (Cummingtonite) (Calcite) (Gibbsite) Solidus 1080 1120 — — temp- erature (° C.) Iron ore A 4 1 0 (mass %) Iron ore B 0 0 2 (mass %) Iron ore C 0 0 0 (mass %) Amphi- 49 0 0 bole group mineral (mass %)

TABLE 4 Iron ore Carbon Melting point adjusting agent (mass %) Silicate Ratio material Silica Amphibole group minerals Yield No. Kind (mass %) (mass %) Limestone Dolomite Fluorite sand mineral (mass %) (%) 1 A 73.2 16.6 5.7 2.8 0.8 — — 2.9 103 2 C 68.8 16.2 11.3 — 0.8 — 2.0 1.0 94 3 B 75.1 18.0 1.9 3.3 0.8 — — — 83 4 B 70.0 18.3 5.4 3.3 0.9 2.0 — — 87 5 C 72.3 17.0 9.0 — 0.8 — — — 88 

1. A method for manufacturing a reduced iron, comprising the steps of: agglomerating a mixture including an iron oxide source, a carbonaceous reducing agent, and a melting point adjusting agent, and heating a resulting agglomerate to reduce an iron oxide in the agglomerate, wherein as the agglomerate, the one containing a silicate mineral with a solidus temperature of 1300° C. or less in an amount of 1 mass % or more is used.
 2. The manufacturing method according to claim 1, wherein the silicate mineral contains a volatile matter.
 3. The manufacturing method according to claim 1, wherein the silicate mineral is of an amphibole group.
 4. The manufacturing method according to claim 3, wherein the amphibole group is at least one or more selected from the group consisting of actinolite, cummingtonite, and grunerite.
 5. The manufacturing method according to claim 1, wherein as the iron oxide source, the one containing a silicate mineral with a solidus temperature of 1300° C. or less is used.
 6. The manufacturing method according to claim 1, wherein as the melting point adjusting agent, a silicate mineral with a solidus temperature of 1300° C. or less is used. 